An interaction model between Cotesia flavipes and Cotesia sesamiae, parasitoids of the gramineous stem-borers at the Kenya Coast
Karuku, Muriithi Simon
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One of the greatest challenges facing the people of sub-Saharan Africa is the production of sufficient food to feed a rapidly increasing population in the face of dwindling finances. As the population grows at 3% and food production at 2% per annum, an annual shortage of 250 million tons of food is expected by year 2020. The greatest obstacle to increasing the production of maize and sorghum, the staple food in many African communities south of the Sahara, is damage by phytophagous insects. Larval feeding in the plant Whorl and later through stem tunneling causes plant damage. Infested plants have poor growth and reduced yield and are more susceptible to secondary infection and wind damage. Estimates of yield losses due to stem-borer are in the neighborhood of 20-40% of the potential yield. To realize the potential of the Gramineae family in ensuring food security in the world, the stem-borers have to be effectively controlled. Various methods have been tried in a bid to control these pests. In biological control, one of the approaches is to find an exotic natural enemy that will successfully fit into the community of existent natural enemies. Hampered by a lack of economic and convenient tools, however, advances in biological control have been largely overshadowed by the rush to exploit insecticides and the ready availability and comparative simplicity of cultural methods. But that is changing. Effects on non-target organisms, resistance development and environmental pollution have incapacitated insecticides and other chemical-based methods. In this study, a simple one host-two parasitoids interaction model with a non-linear trend is developed to predict and understand the reasons for the ultimate impact of the exotic parasitoid Cotesia flavipes (Cameron) (Hymenoptera Braconidae) on stem-borer population dynamics in the coastal area of Kenya. Results indicate that the ultimate extent of suppression of the stem-borers is largely determined by three attributes of the parasitoids namely; the net reproductive rate, the degree of aggregation and the searching efficiency. The model predicts coexistence of all the species considered with C. flavipes dominating the interactive system. Implications of the results for introduction scheme of parasitoids to control pest are discussed. We argue that a model of intermediate complexity may offer the pest prospects of predictive biological control in situations where it is not practicable to obtain the information needed to build and parameterize a large tactical simulation model. The conclusions we reach are of relevance to classical biological control practices, and in particular to those programs in which more than one parasitoid species has been introduced to combat a particular pest of a perennial standing crop system.